Patrick J. Doran and Richard T. Holmes

A nest of the Black-throated Blue Warbler with four nestlings (count the yellow beaks!) on the left and a male of this species feeding nestlings on the right. Black-throated Blue Warblers prefer nesting in areas with a thick layer of understory vegetation, and from year to year, such areas are continually occupied.

Contact Info:
Dr. Patrick J. Doran
Director of Science
The Nature Conservancy in Michigan
101 East Grand River Avenue
Lansing, MI 48906
phone: (517) 316-2279
email: doran@tnc.org

Date Prepared: January 2007

Within a species, the abundance of individuals varies from place to place, even in what may appear to be fairly homogenous habitat, such as the northern hardwoods of the 3160 hectare Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Beginning in the summer of 1999 (and ongoing), we initiated a multi-year study to examine the causes and consequences of variation in the abundance of individuals within the Hubbard Brook watershed (Figure 1).

Figure 1. General physiographic features of the Hubbard Brook Experimental Forest, New Hampshire, USA, showing elevations and drainage patterns. Paired dots represent sample points (n = 371) at which birds abundances were determined.

Using a valley-wide grid of over 370 plots (Figure 1) established in the mid-1990s by Paul Schwarz (See Schwarz et al. 2002), we conducted intensive bird surveys from 1999-2001. Based upon these three years of survey data we found that most bird species exhibited intraspecific spatial variation in abundance (Figure 2). Furthermore, these patterns remained stable across years despite variation in total abundance across years (Figure 3). For five species, spatial variation in abundance was fully explained by spatially varying habitat factors. However, the remaining species continued to exhibit spatial variation in abundance after removing habitat effects, indicating that alternative ecological mechanisms (e.g., conspecific attraction; Stamps 1988) may be operating to increase local abundance beyond that expected based on habitat associations (Doran 2003).

For the black-throated blue warbler (Dendroica caerulescens) one consequence of spatial variation in abundance is that specific locations on the landscape are occupied consistently from year-to-year, while others are occupied intermittently. Consistently occupied sites had more deciduous vegetation and a well-developed dense understory. Warblers were more abundant and arrived earlier in the breeding season at these sites. While there were no differences in average reproductive output across sites with different occupancy rates, these results indicate that the majority of individuals are found, and, subsequently, the majority of young are produced, within relatively few locations on the landscape (Doran and Holmes 2005). Combined, these results indicate that, even within relatively undisturbed, homogenous landscapes, there is significant intraspecific temporal and spatial variation in bird abundance which appears to result from subtle variation in environmental and behavioral factors.

Figure 2. Distribution and abundance of three relatively common species at the Hubbard Brook Experimental Forest . The Black-throated Blue Warbler is a species of primarily deciduous forest; the Blackpoll Warbler found exclusively in high elevation conifer stands; and the Yellow-rumped Warbler found in mixed deciduous-coniferous stands. The size of the green circle represents an index of abundance, with larger dots representing a higher number of singing males detected at a point. In general, points with greater abundance tend to be clumped within the valley.

Figure 3. Distribution and abundance of the Black-throated Blue Warbler from 1999-2001 as well as an overall count of the number of years that the Black-throated Blue Warbler was detected at each survey point across those three years. Symbols are as in Figure 2. Note that areas of high abundance, as well as areas of low abundance, remain in the same general location within the valley across years.

References

Doran, P.J. 2003. Intraspecific spatial variation in bird abundance: patterns and processes. Ph.D. thesis, Dartmouth College, Hanover, NH.

Doran, P.J., and R.T. Holmes. 2005. Habitat occupancy patterns of a forest dwelling songbird: causes and consequences. Canadian Journal of Zoology 83:1297-1305.

Schwarz et al. 2002. Factors controlling spatial variation of tree species abundance in a forested landscape. Ecology 84: 1862-1878.

Stamps, J. A. 1988. Conspecific attraction and aggregation in territorial species. American Naturalist 131:329-347.